Focused ion beam systems are used to fabricate or alter microscopic or nanoscopic structures. Some focused ion beam (FIB) columns use a plasma ion source, such as the inductively coupled plasma (ICP) source described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention. Focused ion beam systems using plasma ion sources, such as that described above, have several advantages over systems using liquid metal ion sources. For example, plasma ion sources can provide a higher brightness with reduced energy spread. Plasma ion sources can provide a variety of ion species, some of which have higher sputter yields and do not contaminate the sample.
FIG. 1 shows schematically an ICP system according to the prior art that is described in U.S. Pat. No. 7,241,361. System 100 includes an ion beam column 102 with a plasma ion source including a plasma chamber 104 which is supplied with gas or gases through a capillary or flow restrictor through gas inlet 106. A coil 107 is coupled by an impedance matching circuit to an RF source, not shown, to supply energy to ionize the gas in the plasma chamber 104. System 100 preferably includes a means to reduce the energy spread of ions in the ion beam. Such a means can include a split Faraday shield to reduce capacitive coupling between the antenna 107 and the plasma or a balanced antenna. Ionized gas atoms or molecules are extracted from plasma chamber 104 by an extraction electrode 105 that pulls ions through an aperture 108 that serves as a source electrode that electrically biases to the plasma to a high potential, that is, greater than 10,000 V. Ions are accelerated toward a work piece 110 positioned on an adjustable stage 112. Ion beam systems also typically include a beam blanker 130 for blanking the ion beam, beam deflectors 132 for positioning the beam, and a focusing lens 134 to collimate or focus the beam of ionized molecules.
Typically, only a very small percent of the atoms or molecules in the plasma chamber are ionized. Neutral atom can diffuse through the aperture from which the ions are extracted. The neutral atoms, however, have very low energy because they are not accelerated by the extraction electrode. It has been thought that very few of the low energy neutral atoms reach the work piece, because they diffuse from the aperture in random directions, collide with elements of the optical column, and are removed by the vacuum pump.
Focused ion beam systems are also used to form microscopic structures by etching material from a work piece or depositing material onto a work piece. The ions can remove material from the surface by sputtering, that is, momentum transfer from the ion to atoms in the work piece. The ions can also activate a precursor gas that decomposes in the presence of the ion beam to deposit a material or to form a volatile compound with the target material to enhance etching of the target. Ion beam systems often also include a gas inlet for injecting precursor gases 140 and a needle 142 for directing the flow of precursor gases toward the work piece surface.
Focused ion beam system also typically include a secondary electron detector 150, such as an Everhart-Thornley detector, for forming an image as the ion beam scans the sample surface and the ion beam impact produces secondary electrons. The image contrast at each point is determined by the number of secondary ions detected. Applicants have found that secondary electron images formed by the ion beam from a plasma source can have unexpectedly poor contrast. An improved apparatus and method to collect higher quality secondary electron images and to produce higher resolution etching or deposition is therefore needed.